Pyrolysis of organic materials is an important technique in processing feed material with a high organic content to obtain gaseous and condensable fuels. Pyrolysis of biomass materials is of increasing interest because it can produce high yields of combustible products and is considered a “green” (low-polluting, environmentally friendly) process.
Pyrolysis is a process whereby organic containing feed materials are heated in a reducing or inert environment to produce combustible organic vapors (oils, tars, etc), non-condensable gases, including synthesis gas (“syngas”), as well as char and ash. “Fast Pyrolysis” is a pyrolysis technique in which the organic feed material is heated very rapidly and exposed to elevated temperatures for relatively short time periods (less than three seconds). Fast pyrolysis usually results in greater yields of condensable oils compared to other pyrolysis techniques. However, it is very difficult to contact high temperature abrasive inert solids with a low temperature, single phase (solid or liquid) or two phase (liquid/solid) reactive material to rapidly obtain a physically and chemically homogeneous mixture.
One method of fast pyrolysis involves mixing of an ambient-temperature feed material, such as coal, biomass, or another organic material with inert hot solids, such as silica, olivine, alumina, or other materials that may be available. It is desirable that this mixing occurs in an environment with little or no oxygen, and that thorough mixing producing an essentially uniform mixture occurs very rapidly.
When this process is correctly controlled, free moisture is evaporated and most of the high molecular weight organics within the feed are “cracked” or volatilized to lower molecular weight compounds that are in the vapor phase at the reaction temperature. The balance of the material remains as a solid in the form of a very high carbon content, substantially inorganic char/ash. This fast pyrolysis process requires rapid mixing of different solids. However, the characteristics of the materials to be mixed, such as temperature, density, size distribution, etc., may be quite different. Because the resulting mixture must be substantially uniform, rapid mixing may be difficult to achieve in the mixing device.
Further, the residence times and temperatures of the solids and gases resulting from the pyrolysis within the mixing device can greatly affect the result. For example, short residence times of less than three seconds and temperatures between approximately 700 and 1100 degrees Fahrenheit promote formation of organics that may be condensed and recovered as liquids. This is an example of “fast pyrolysis,” of feed material to produce organic liquids for use in power generation or for the production of transportation fuels. Longer residence times of over five seconds at higher temperatures in the range of approximately 1300 degrees Fahrenheit or more promote additional cracking resulting in a higher yield of low molecular weight products (gasification), and formation of reduced quantities of condensable organics.
There have been prior attempts to efficiently implement fast pyrolysis. For example, U.S. Pat. Nos. 5,792,340 and 5,961,786 to Freel, et al. disclose the use of a mixing section for rapidly mixing carbonaceous feedstock and an inorganic particulate heat medium while using transport gas to entrain the mixture into a reactor section for pyrolysis. However, these patents disclose a process with a limited heat carrier density, which is maintained within 4.5 to 18.6 million particles per cubic foot of reactor volume. Moreover, this process requires the use of transport gas to move the mixture from the mixing section through the reactor.
Accordingly, it is desirable to provide a process for fast pyrolysis or gasification that will provide efficient conversion of feed stocks and avoid the use of transport gas.
It is further desirable to provide a process for fast pyrolysis or gasification with applicability to emerging fields of alternate energy production (e.g., pyrolysis of biomass), as well as classical refining operations (e.g., fluidized catalytic cracking).